For nearly 70 years, physicists working on magnetic data storage operated with a number no one had actually verified. The assumption was that nanomagnets attempt to flip their magnetic direction roughly once per nanosecond. A new study from Tohoku University has now measured that rate directly for the first time, and the real number is strikingly different.
The research, published in Communications Materials, found that the attempt time is between 4 and 11 nanoseconds. That makes the actual switching attempts more than ten times slower than the long-standing assumption baked into storage device design and theory.
The stakes are practical. Hard drives store each bit of data in a tiny magnet. That magnet needs to stay pointing in one direction long enough to preserve the information. The stability of that direction depends on an energy barrier the magnet must overcome to flip. As storage devices pack more data into less space, the individual magnets shrink, and a smaller magnet has a lower energy barrier. That makes it more vulnerable to random thermal noise flipping it accidentally, which would corrupt or erase stored data.
The probability that such a flip occurs follows a well-established equation called the Arrhenius law. Within that framework, a key parameter called the attempt time, denoted τ₀, describes how frequently a magnet tries to cross the energy barrier. If the attempt time is short, the magnet is making many more attempts per second and is statistically more likely to flip. If the attempt time is longer, the magnet is more stable than models predicted.
The problem was that no one had directly measured this parameter. "This parameter has been assumed for decades but had never been directly measured," said Shun Kanai, associate professor at the Research Institute of Electrical Communication at Tohoku University. "Our experiments show that the fundamental switching attempts of nanomagnets occur much more slowly than previously thought."
Getting to that measurement required building specialized nanomagnet devices and characterizing their physical geometry using scanning electron microscopy. The team then developed a new experimental and analytical approach that tests the Arrhenius law without needing to change the temperature, which had been a barrier to previous measurement attempts. By observing how the devices switched between two opposite magnetization states at room temperature, they could extract the attempt time directly.
The result has real consequences for how engineers model and design magnetic storage. If nanomagnets are more stable than previously calculated, that affects predictions about how small storage elements can be made before data loss becomes likely, and potentially how long data written to a device can be expected to survive under normal operating conditions.
The finding also raises a more fundamental question: if one of the basic parameters in nanomagnet physics was off by a factor of ten for 70 years, it invites scrutiny of other long-standing assumptions in the field that have similarly never been put to a direct experimental test.
